1. Field of the Invention
The present invention relates to an implantable heart stimulator of the type having means for setting a maximum allowable stimulation rate, and an ischemia detector.
2. Description of the Prior Art and Related Subject Matter
Ischemia is a condition resulting from insufficient blood flow through the heart muscle. The reason therefor is blocking or passage congestion of coronary blood vessels of the heart. Blood penetration of the heart muscle is possible only in the diastolic phase, that is the phase between two consecutive contractions of the heart, when the aortic valve is closed. About 60% of the oxygen content inside the heart tissue is consumed during a heart contraction and in order to maintain the pumping efficiency of the heart the consumed oxygen must be refilled until the next contraction.
An increased heart rate results in only minor shortening of the systolic phase, that is the contraction phase of the heart, and consequently an increased heart rate results mainly in a shortening of the diastolic phase, which is the period during which oxygen is supplied to the heart as mentioned above. An increased workload will consequently worsen the situation for an ischemic patient.
In such a situation a symptomatic (episodic) ischemia, that is angina pectoris, heart insufficiency or infarct, will force the patient, because of the associated pain, to stillness, with a reduced heart rate as a consequence.
No heart stimulators, like pacemakers, are able to react to pain, and so called rate response pacemaker systems responding to metabolical, hemodynamical or activity inputs will try to compensate for the oxygen deficiency of the heart by increasing the stimulation rate, thus worsening the ischemic situation of the patient.
In U.S. Pat. No. 5,199,428 a technique is described for detecting ischemia and both effecting stimulation of nerves regulating blood pressure and heart rate to reduce the heart's oxygen requirements while providing pacing therapies to maintain the patient's heart rate within acceptable limits, e.g. to overcome bradyarrhythmia and/or unphysiological AV-delays induced by the nerve stimulation.
A large portion of cardiac ischemia is silent. It has been suggested that up to 80% of ischemic heart diseases are silent, i.e. a state of ischemia which the patient is not aware of. Rate modulated pacing is used to override the poor sinoatrial response of the patient to exercise which, in this case, is physiological.
It has also been proposed to provide heart stimulators provided with an ischemia detector to lower the actual stimulation rate in response to the detection of an ischemic episode, in order to slow down or stop the further development of the ischemia (see Swedish Patent Application SE 9700182-0, filed Jan. 23, 1997, or corresponding PCT application PCT/SE98/0043, filed Jan. 13, 1998, both of these applications being unpublished as of the filing date for the present application.
In rate modulated pacing systems a programmable value sets the highest pacing rate that can be achieved in response to sensor input, the so-called maximum sensor rate (MSR). In an ischemic situation a sensor which is sensing e.g. the physical activity level of the patient, e.g. an accelerometer, can often increase the stimulation rate, as discussed above. When a sensor is controlling the pacing rate in this way, the pacing rate thus will not exceed the programmed maximum sensor rate for the sensed level of activity.
In a dual-chamber sensing and tracking pacing system in which the ventricle is stimulated with a certain delay in response to a detected atrial activity normally a programmable value sets the highest allowable ventricular pacing rate, the so-called maximum tracking rate (MTR) Conventionally the maximum sensor rate and the maximum tracking rate are set so high that in an ischemic situation the pacing system can stimulate the heart to an infarct.